1. Field of the Invention
The present invention relates to a magnetooptical recording medium having a four-layered structure and capable of performing an over-write operation by modulating only an intensity of a light beam in accordance with information to be recorded without modulating a direction and strength of a bias field Hb.
2. Related Background Art
In recent years, many efforts have been made to develop an optical recording/reproduction method which can satisfy various requirements including high density, large capacity, high access speed, and high recording/reproduction speed, and a recording apparatus, a reproduction apparatus, and a recording medium used therefor.
Of various optical recording/reproduction methods, the magnetooptical recording/reproduction method is most attractive due to its unique advantages in that information can be erased after it is recorded, and new information can be repetitively recorded.
A recording medium used in the magnetooptical recording/reproduction method has a perpendicular magnetic layer or layers as a recording layer. The magnetic layer comprises, for example, amorphous GdFe or GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentrical or spiral tracks are normally formed on the recording layer, and information is recorded on the tracks. In this specification, one of "upward" and "downward" directions of magnetization with respect to a film surface is defined as an "A direction", and the other one is defined as a "non-A direction". Information to be recorded is binary-coded in advance, and is recorded by two signals, i.e., a bit (B.sub.1) having an "A-directed" magnetization, and a bit (B.sub.0) having a "non-A-directed" magnetization These bits B.sub.1 and B.sub.0 correspond to "1" and "0" levels of a digital signal. However, in general, the direction of magnetization of the recording tracks can be aligned in the "non-A direction" by applying a strong external field before recording. This "aligning process" is called "initialize*" in a conventional sense. Thereafter, a bit (B.sub.1) having an "A-directed" magnetization is formed on the tracks. Information is expressed in accordance with the presence/absence and/or a bit length of the bit (B.sub.1). Note that a bit is often called a mark recently.